1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, for verification of the spacing between read/write heads and a magnetic recording disk within a magnetic disk drive system while in end-user operation.
2. Statement of the Problem
Many computer systems use magnetic disk drive systems for mass storage of information. Magnetic disk drive systems typically include one or more sliders that include a read head and a write head. An actuator/suspension arm applies a slight spring force to position the slider proximate to a magnetic recording disk. The side of the slider facing the magnetic recording disk is called an Air Bearing Surface (ABS). When the magnetic recording disk rotates, air flow generated by the rotation creates an air bearing between the slider ABS and the magnetic recording disk. This air bearing causes the ABS of the slider to fly at a particular height above the magnetic recording disk, which is also called the fly height. As the slider flies on the air bearing, a voice coil motor moves the actuator/suspension arm in a radial direction along the surface of the magnetic recording disk. This radial movement allows positioning of the slider over selected tracks of the magnetic recording disk to allow reading and writing of data by the read/write heads.
One factor that contributes to the effective reading and writing of data by the read/write heads is the spacing of the read/write heads in relation to the surface of the magnetic recording disk. This spacing generally depends on the fly height of the slider, which is determined by the ABS of the slider. As areal densities of magnetic recording disks increase, it becomes more important to precisely control the spacing of the read/write heads in relation to the magnetic recording disk, as the spacing may be 10 nanometers or less.
To further control the spacing between the read/write heads and the magnetic recording disk, some sliders include heating elements which are fabricated proximate to the read/write heads. The read/write heads are fabricated from materials that have a different thermal rate of expansion than the body of the slider. When a current is applied to the heating elements, the read/write heads protrude from the ABS of the slider towards the surface of the magnetic recording disk. This protrusion reduces the spacing between the read/write heads and the magnetic recording disk. The use of heating elements, which is also referred to as thermal fly height control, allows for a more precise control of the spacing between the read/write heads and the magnetic recording disk.
After manufacturing, but before being shipped to an end-user, a magnetic disk drive system is calibrated to generate a desired spacing between the read/write heads and the magnetic recording disk. To calibrate a magnetic disk drive system, a control system applies a motor current to a spindle motor which in turn rotates a spindle connected to the magnetic recording disk. As the magnetic recording disk rotates, the slider flies over the surface of the magnetic recording disk on the air bearing. A thermal fly height controller then incrementally increases the heating power applied to the heating elements in the slider in order to increase the protrusion of the read/write heads towards the surface of the magnetic recording disk. At some threshold of heating power applied to the heating elements in the slider (herein referred to as the contact power), the read/write heads will contact the surface of the magnetic recording disk. Based on the spacing due to the contact power and the spacing without heating power applied, a relationship may be determined between the heating power and the spacing between the read/write heads and the magnetic recording disk. Magnetic disk drive system manufacturers may then use this relationship to define an operating power applied to the heating elements which results in the desired spacing between the read/write heads and the magnetic recording disk. For example, if the spacing between the read/write heads and the magnetic recording disk without any heating power applied is 10 nanometers (nm), and the calibration process determines that the contact power applied to reduce the spacing to zero is 100 milliwatts (mW), this would yield a relationship of 10 mW/nm (assuming a substantially linear relationship). In order to generate a desired spacing of 6 nm, an operating power of 40 mW would be applied to the heating elements in the slider to cause a 4 nm thermal protrusion of the read/write heads towards the magnetic recording disk.
After calibration, the magnetic disk drive system is shipped to an end-user. When in end-user operation, the magnetic disk drive system uses the operating power to generate the desired spacing between the read/write heads and the magnetic recording disk. As the drive ages, and due to changes in environmental conditions, the operating power applied may be inadequate to generate the desired spacing, which may lead to reduced performance or potential drive failure due to head to disk contact. Head to disk contact occurs when the spacing between the read/write heads and the magnetic recording disk is reduced until the ABS of the read/write heads contact the magnetic recording disk. Head to disk contact is undesirable, as it may cause damage to the slider or to the magnetic recording disk.